1. Field of the Invention
The present invention relates to a method of manufacturing an insulating substrate which can secure an insulation within a package such as a power semiconductor apparatus, and more particularly to an insulating circuit board intended to improve an insulating property of a composite member comprising an insulating substrate and an electrode conductor bonded to the insulating substrate by a bonding member, and a power semiconductor apparatus employing the insulating circuit board.
2. Description of the prior Art
Conventionally, there has been known a semiconductor apparatus which is composed by sealing a semiconductor device, such as an IGBT, a diode, a GTO, a transistor or the like, within an insulating container. The apparatus is applied to various kinds of inverter apparatus or the like in accordance with its voltage resistance property and its current capacity. The apparatus is composed so as to insulate an electric circuit from a base via an insulating substrate such as an alumina (Al2O3), an aluminum nitride (AlN) or the like within the apparatus, thereby apparatus. In particular, in the insulation between the base and the electric circuit, a withstand voltage in 10 kV class has become required. In order to solve the requirement, an average electric field strength is reduced by, in general, a method of increasing a thickness of the insulating substrate so as to reduce the electric field strength, or a method of increasing a creepage distance between the circuit pattern and an end portion of the insulating substrate.
In order to improve the withstand voltage, it is necessary to reduce a local electric field strength which causes an insulation deterioration. In many cases of using the insulating circuit board in the semiconductor apparatus, a whole of the insulating circuit board is covered with an organic resin such as a silicone gel. In the structure covered with the resin, a partial electric discharge is generated in an electric field concentrating portion having a local great electric field strength as a precursor phenomenon of a dielectric breakdown, thereby a void or an electric tree is generated. In the case that a voltage is continuously charged, the electric discharge is continuously generated, so that there is a risk that the tree grows to generate the dielectric breakdown. In the case that a voltage is applied within the semiconductor apparatus, a place in which the electric improving easiness of mounting. Among these devices, the IGBT has advantages of an easy control and a high frequency operation with a great amount of current, since the IGBT is a voltage control type device. In recent years, the IGBT device is developed to have a great capacity, so that the performance thereof is improved up to a field covered by the conventional GTO. In accordance with the increase of the capacity of the apparatus, an area of the insulating substrate used within the apparatus becomes large due to an increase of size and an increases of the number of chips of the semiconductor device used within the apparatus. In order to solve the problem of the increase of the area, there has been employed a method of providing a terraced portion (a step portion) in an end portion of a conductive layer bonded to the insulating substrate as shown in Japanese Patent Publication No. 5-152461, a method of placing an end portion of a bonding member for bonding a electrode conductor and an insulating substrate outside the electrode conductor as shown in Japanese Patent Publication No. JP-B2-7-77989, or the like. These methods reduce a maximum stress generated in the electrode conductor or between the bonding member and the insulating substrate so that a crack generated by a heat cycle of the insulating substrate is restricted. Further, it is necessary to increase a withstand voltage within the apparatus in correspondence to a high withstand voltage of the field is concentrated is in a creepage portion of the insulating circuit board, more particularly a terminal end portion of the electrode conductor in the creepage portion of the insulating substrate. However, in the method of making the shape of the electrode conductor terminal end portion thin and the method of arranging the bonding member outside the electrode conductor which are employed as a countermeasure against the crack of the insulating substrate in the prior art mentioned above, no consideration is given to a relation between the shape of the terminal end portion of the electrode conductor and the partial electric discharge starting voltage, between the electrode conductors (in the case of applying a voltage between the electrodes in the circuit side) and between the electrode conductor and the electrode on a back surface of the insulating substrate (in the case of applying a voltage between the electric circuit and the base).
On the contrary, in the method of making the insulating substrate thick in order to reduce the average electric field strength, since the heat conduction of the insulating substrate is smaller than that of the electrode conductor (for example, a thermal conductivity coefficient of a copper generally used as an electrode conductor is 398 W/mK and on the contrary, those of an alumina and AlN used as an insulating substrate are 36 W/mK and 175 W/mK respectively, which are one half or less of that of a copper), a heat resistance of the apparatus is increased. Further, the method of increasing the creepage distance so as to improve the withstand voltage makes the apparatus large.
It is an object of the present invention to provide an insulating circuit board having a high withstand voltage (corresponding to a partial electric discharge starting voltage and a breakdown voltage), and to provide a power semiconductor apparatus having a high insulting reliability by using the insulating circuit board.
In order to achieve the object mentioned above, according to the present invention, there is provided a method of manufacturing an insulating circuit board having an insulating substrate, and an electrode conductor bonded to a surface of the insulating substrate by a bonding member to form a circuit pattern on the surface of the insulating substrate, comprising the step of allowing a conductor end portion of the circuit pattern to discharge electricity in an atmospheric or depressurized gas so as to melt and re-solidify the conductor end portion of the circuit pattern.
In the method, an alternating voltage may be applied to the circuit pattern in an atmospheric or depressurized gas so as to allow the conductor end portion to discharge electricity at substantially each of cycles.
Otherwise, a direct voltage may be applied to the circuit pattern in the atmospheric or depressurized gas so as to allow the conductor end portion to discharge electricity at one or more times per one second.
In the case of allowing the conductor end portion to discharge electricity in an atmospheric or depressurized gas, a voltage may be applied to the circuit pattern by using a circuit having a resistance connected to the circuit pattern in series so as to allow the conductor end portion to discharge electricity.
Otherwise, the conductor end portion of the circuit pattern may be melted and re-solidified by laser beam irradiation.
As a result of these methods, the temperature rises due to an electric discharge energy in a portion in which an electric discharge is generated by a concentration of an electric field due to an acute angle shape in the end portion of the electrode conductor, or due to an energy of the laser in the laser irradiated portion, thereby the conductor metal in the end portion of the electrode conductor is melted so as to deform the acute angle shape in the end portion of the electrode conductor and form a smooth shape. In particular, in the case of viewing an upper surface of the insulating circuit board, uneven portions (comprising capes and recesses) having the acute angle shape in the end portion of the electrode conductor are reduced by the melting and re-solidifying, and in the case of viewing a cross section of the insulating circuit board, a curvature radius of a projection shape in the end portion of the electrode conductor is enlarged by the melting and re-solidifying. As a result, it is possible to prevent the electric field form concentrating in the end portion of the electrode conductor, so that the starting voltage for generating the partial electric discharge is increased.
Since it is possible to generate an electric discharge at the voltage lower than the withstand voltage of the semiconductor device by applying the voltage in the atmospheric and depressurized gas, it is possible to apply the voltage and generate the electric discharge even at a time of assembling the semiconductor apparatus as well as the sole insulating circuit board. In that case, the voltage may be applied and electrically discharged before coating an organic resin such as a silicone gel or the like. Further, since the electric discharge energy is concentrated near the electric discharge portion even in the case of the electric discharge at a relatively low voltage, it is possible to obtain an effect sufficient to melt and deform the conductor metal.
A description will be given below of an embodiment in accordance with the present invention and a comparative embodiment with reference to the accompanying drawings.